标题： 压力诱导的过渡金属二硫化物中超流密度的增强，有和没有电荷密度波的情况 显示英文标题

标题： Pressure-Induced Enhancement of Superfluid Density in Transition Metal Dichalcogenides with and without Charge Density Wave

摘要： 深入了解过渡金属二硫属化物（TMDs）中电荷密度波（CDW）序与超导性之间的相互作用，尤其是在(4H/2H)-NbX$_{2}$（X=Se,S）家族中，仍然是一个开放且引人入胜的挑战。因此，需要对这个家族中的各种化合物进行系统微观研究，以揭示这种复杂的相互作用。在这里，我们报告了μ子自旋旋转和磁输运实验，研究静水压力对超导转变温度（$T_{\rm c}$）、温度依赖的磁穿透深度（$\lambda_\mathrm{eff}$）以及电荷密度波序（CDW）的影响，研究对象是两种层状硫属化物超导体：4H-NbSe$_{2}$，它表现出CDW序；2H-NbS$_{2}$，它没有这种序。 我们的观察结果揭示了在最大施加压力为2 GPa时，4H-NbSe$_{2}$的超流体密度（$n_{s}/m^{*}$）有显著的75$\%$增加，超过了2H-NbSe$_{2}$的值。尽管没有CDW序，但在1.8 GPa的施加压力下，对于2H-NbS$_{2}$也观察到了明显的20$\%$增长。 值得注意的是，所有这些TMDs中评估的超导能隙在施加压力变化时基本不受影响，无论压力是否导致(4H/2H)-NbSe$_{2}$中CDW序的部分抑制，或者在2H-NbS$_{2}$中CDW序的一般缺失。 这些结果强调了这些TMDs中压力诱导行为的复杂性，挑战了仅将CDW序与超导性之间竞争视为简单观点的看法。 值得注意的是，$n_{s}/m^{*}$与$T_{\rm c}$之间的关系表现出非传统的相关性，表明与在铜基、kagome和铁基超导体中观察到的行为有显著相似之处。 显示英文摘要

摘要： Gaining a deeper understanding of the interplay between charge density wave (CDW) order and superconductivity in transition metal dichalcogenides (TMDs), particularly within the (4H/2H)-NbX$_{2}$ (X=Se,S) family, remains an open and intriguing challenge. A systematic microscopic study across various compounds in this family is therefore required to unravel this complex interplay. Here, we report on muon spin rotation and magnetotransport experiments investigating the effects of hydrostatic pressure on the superconducting transition temperature ($T_{\rm c}$), the temperature-dependent magnetic penetration depth ($\lambda_\mathrm{eff}$), and the charge density wave order (CDW) in two layered chalcogenide superconductors: 4H-NbSe$_{2}$, which exhibits CDW order, and 2H-NbS$_{2}$, which lacks such order. Our observations reveal a substantial 75$\%$ enhancement of the superfluid density ($n_{s}/m^{*}$) in 4H-NbSe$_{2}$ upon maximum applied pressure of 2 GPa, surpassing that of 2H-NbSe$_{2}$. Despite the absence of CDW order, a sizeable 20$\%$ growth in superfluid density is also observed for 2H-NbS$_{2}$ under an applied pressure of 1.8 GPa. Notably, the evaluated superconducting gaps in all these TMDs remain largely unaffected by changes in applied pressure, irrespective of pressure-induced partial suppression of CDW order in (4H/2H)-NbSe$_{2}$ or its general absence in 2H-NbS$_{2}$. These results underscore the complex nature of pressure-induced behaviors in these TMDs, challenging a simplistic view of competition solely between CDW order and superconductivity. Remarkably, the relationship between $n_{s}/m^{*}$ and $T_{\rm c}$ exhibits an unconventional correlation, indicating a noteworthy similarity with the behavior observed in cuprate, kagome, and iron-based superconductors.